An Allegory of Soil Classification

In the kingdom of Terra Firma, there exists competition among excavated soils to see which can achieve the highest possible ranking in terms of soil stability. A variety of soil types go into battle against the common enemy, “Site Conditions”. These battles between soils and site conditions are ongoing in a world under the influence of constant gravitational forces.

Within the family of soil types, some have an inherent advantage over other soils. For example, STABLE ROCK, such as granite, has a distinct advantage and automatically gets a pass. STABLE ROCK is an igneous rock, formed by heat and the particles are welded together by nature. Because of its structure and beauty, granite is often used in building materials. When excavated, this solid mass of granite rock will stand indefinitely without the need for support. Therefore, solid stable rock is born and lives on the finish line, having achieved the noble status of “STABLE ROCK” at birth.

While only a select few of these rock types are destined (or predestined) to achieve the lofty position of STABLE ROCK, all other soils compete to seek their potential. In the end each soil type will be ranked according to their stability. The ranking is based on the soil’s ability to fight the enemy, in a world under the influence of gravity. The rankings, from highest to lowest are as follows:

TYPE A – More stable

TYPE B

TYPE C – Least stable

While they potentially compete against the same obstacles, a particular soil’s ability to perform and achieve a rank varies to a great degree due to its structure and composition.

In Terra Firma, soils belong to one of five “Houses” or groupings. These are:

·??????Rock

·??????Granular

·??????Cohesive

·??????Cemented

·??????Loam

The stability of these soils and how they rank is not in a vacuum. These soils are subject to conflict with the enemy which can be the result of both natural and man-made conditions. The enemy may be altogether absent, or may be present to some degree, possibly in full force.

?The Enemy of Soil Stability “Site Conditions”

·??????Fissures

·??????Previously disturbed

·??????Vibrations

·??????Water

·??????Layers dipping into the excavation on a 4:1 ratio

?The House of Rock.?As described, STABLE ROCK is the most noble of all soil types due to its strength and its ability to stand against any enemy attack.

However, STABLE ROCK has a distant relative known as UNSTABLE ROCK, which although it also belongs to the house of rock, it is not born with supernatural strength. Therefore, UNSTABLE ROCK has limited potential and at best can achieve a stability level of?TYPE B.

If UNSTABLE ROCK happens to be submerged under water, the stability rating drops dramatically down to a?TYPE C.

?The House of Granular Soils.?From the house of granular soils, we have siblings: GRAVEL, SAND, and SILT.

Poor SAND is weak, unstable, and does not do well in competition. SAND is always and forever doomed to the lowest level,?TYPE C.

GRAVEL is SAND’s bigger brother. Interestingly, there are two distinctive lines branching from the GRAVEL family; Angular GRAVEL and Round GRAVEL. The Round GRAVEL, like SAND, is doomed to the lowest level,?Type C.

Angular GRAVEL, however, is more stable because it is shaped like crushed rock and therefore there is more friction between particles. This friction enables it to stand at a better angle than Round GRAVEL particles. Angular GRAVEL has the potential to achieve the second level,?Type B.

At the other end of the granular spectrum are the tiny SILT particles. Due to their extremely small size, and the greater surface area contact, these particles also have the surprising potential to achieve the second level,?Type B.

However, the enemy known as water, has an easy time of destabilizing soils from the house of granular. If any soils from the house of granular are submerged or if water is freely seeping through them, their stability plummets to the lowest level,?TYPE C.

?The House of Clay.?Another completely different family of soils are the CLAYS. This group is also divided into three subcategories. They are strong CLAY, medium CLAY, and weak CLAY.

Strong CLAY has the potential to reach the high-level rating of?TYPE A. The only problem is that this level can easily be reduced to?TYPE B?when under attack by certain site conditions. For example, if this strong clay has fissures, or if it has been excavated before, or even if it is subject to vibrations, the rating is reduced to?TYPE B.

Medium CLAY has the potential to reach as high as the?TYPE B?level. When dry, this soil possesses an admirable stability rating.

Unfortunately, water has a profound effect on this soil type, reducing both the?Type A?or?Type B?stability rating all the way down to a?TYPE C?if it is either submerged or is freely seeping water.

Unfortunately for weak clay, it is another soil that is doomed to the lowest level of?TYPE C, regardless of any enemy presence.

?The House of Cemented Soils. A distant relative of cohesive and granular soils are the CEMENTED soils. These are commonly known as caliche and hardpan, and they have a structure that allows them to compete for the high level of?Type A.

Like Strong CLAY though, even though CEMENTED soil has the potential to reach the high level of?TYPE A, this level can easily be reduced to?TYPE B?when attacked by site conditions such as fissures, or if it has been excavated before, or if it is subject to vibrations. If so, CEMENTED soil also has its rating reduced to?TYPE B.

When dry, this soil also possesses an admirable stability rating, but the stability will be reduced to a?TYPE C?if it is submerged or is freely seeping water.

?The House of Loam.?On the surface this house is filled with mystery and questions. It’s only when one opens the door and meets with the citizenry that the answers begin to appear. The loam family turns out to be a combination of soils from two other houses, the house of granular and the house of clay. This is a very large family and is distinctive only by the proportion of gravel, sand, silt, and clay. For example, if a member of the house of loam has enough clay in its DNA, it will tend to act like clay, and for all practical purposes easily poses and passes as someone from the house of clay. Likewise, the same can be said for a member of this house that is predominantly made of granular material. It will tend to act like someone from the house of granular, and its stability rating will be judged accordingly.

Part of the mystery from the house of loam is how inherently stable these soils tend to be. For example, there are 5 loams mentioned in the definition of?Type A?soil. There are 4 loams mention in the definition of?Type B?soil. There is only 1 loam mentioned in the definition of?Type C?soil. If 9 of 10 loams are either Type A or Type B, that means loams generally tend to do well in the arena of stability rankings.

As with soils from the house of clay, any?Type A?loam that is subjected to fissures, have been previously disturbed, or is subject to vibrations are automatically downgraded to a?Type B?classification.

Also, if any?Type A?or?Type B?loams are under water or are freely seeping water, they are downgraded to a?Type C?stability rating.

The one loam that is born with a?Type C?rating is known as a Loamy Sand. As one might guess, this loam is predominately composed of sand.

?The Case of Soil Layers

In Terra Firma, the soils from different houses are sometimes naturally and randomly layered on top of one another. There could easily be soils from the house of granular positioned over a layer of soil from the house of clay. When these soils are excavated, the stability of the layers depends on if the stronger layer is above or below the weaker layer. If the stronger layer is underneath the weaker layer of soil, the stability ratings may be done independently. In other words, there could be a?Type C?soil in a layer above a?Type B?layer and the excavation could have two distinct stability rankings.

However, if the converse occurs, such as a?Type B?layer positioned over a?Type C?layer, the entire excavation must be treated as a?Type C?because the weaker layer is on the bottom.

?The Case of Sloped Soil Layers

In some parts of Terra Firma, there are some rare situations where the soil has been subjected to upheaval or shifting which has resulted in the soil layers being tilted on an angle, relative to the earth’s surface. When excavating through these layers, sometimes the direction of the excavation can be such that the layers slope toward the open excavation. If this happens, and if the layers are sloped steeply enough, gravitation forces would be pulling the “uphill” side in toward the open excavation.

This relates to the previous discussion on classifying soil layers; however, the twist is, that if the layers sloping toward the open excavation are steep enough, it doesn’t matter if the stronger layer is on the bottom or not. If the layers slope toward the open excavation on a slope of four to one, the classification must be a?Type C, regardless of whether or not the stronger layer is below the weaker layer. For example, if the excavation is four feet wide, and the sloping layers show a difference of one foot from one side compared to the opposite side, that would constitute a four to one ratio. Similarly, if the excavation is six feet wide and there is a difference between the layers of at least 18 inches, that would also constitute a four to one ratio and a classification of?Type C.

?The Unique Case of C-60 soil

There is redemption for some soils, but not all, that fall into the?Type C?category. For example, if a soil is?Type C?but is stable enough to stand when excavated long enough for a protective system to be installed; and, if that soil is not subject to water influence, it can potentially be deemed?C-60?which is more stable than the worse case?C-80.?This distinction has important ramifications when it comes to protective system options in a?Type C?soil scenario.

Conclusion

A discussion of soil classification is incomplete if the reason “why” is not included. There is only one reason to classify soils and that is to ensure that an adequate protective system is in place to protect employees. Whether using a shoring or shield system, or if sloping or benching is the protective system of choice, whether it is adequate and how it can be used comes down to the quality of soil and the soil stability ranking.